Method for molding ophthalmic lenses

ABSTRACT

The invention concerns a method for moulding a lens, between a first mould ( 2 ) and a second mould ( 6 ) which consists in: machining at least one of the moulds ( 2 ) into the shape of the lens to be obtained, before moulding. The invention enables to obtain finer lenses, while limiting the required amount of hardenable material for making the lens.

The invention relates to molded lenses, and more specifically to amethod for molding ophthalmic lenses.

Ophthalmic lenses made of organic glass—polymerized material—are usedfor optical correction; they are mounted for this purpose in frames. Oneof the problems which arises for such lenses is that of theirmanufacture.

CA-A-0 596 660 discloses a method and an apparatus for molding lensesmade of setting resin. Said document proposes to mold the lenses betweentwo tempered glass molds with parallel faces; the molds are circular,and are separated at their periphery by a flexible or rigid ring sealwhose thickness corresponds to the final thickness of the lens. Theassembly allows the lenses to be molded without the resin coming intocontact with the air when it is heated. Said document mentions theproblems of shrinkage of the resin when it is heated, and of isolationof the ambient air for the polymerization. FR-A-1 128 971 proposes theuse of quartz molds for the molding.

FR-A-1 166 582 discloses a method and apparatus for molding lenses; theapparatus has a lower punch, made of glass and/or metal, and an upperpunch; said document suggests that the lower punch should extend beyondthe edge of the upper punch, so as to make a reserve for an excess ofmonomer to be polymerized. As the document proposes heating the monomerstarting from the center of the punch, the presence of excess resinmakes it possible to compensate for the shrinkage in the region of themolding surfaces of the punches. Said document also mentions that therim of the lens is rough due to the provision of the reserve, and mustthen be polished.

U.S. Pat. No. 5,288,221 proposes a solution to another problem. Thesubject of said document is that of solving the problems arising in themanufacture of ophthalmic lenses by bonding two wafers to each other.This bonding is an operation which is difficult to perform especially onaccount of the impurities and bubbles in the adhesive; in addition, itis necessary in such a solution to have available a large number ofwafers. Said document thus proposes to mold a lens to one of the wafers:during the polymerization, the polymerizable material assembles with thewafer to make the lens. To carry out the molding, said US documentproposes to use a ring seal, arranged around the wafer. A supportpositions the mold at a known distance from the wafer, so as to be ableto cure the polymerizable material. It is also suggested in saiddocument that the mold should be slightly smaller than the wafer: thismakes it possible to fill the cavity to excess to avoid bubbles, andthat the excess material should escape between the periphery of the moldand the ring seal. EP-A-0 371 621 describes another method formanufacturing lenses by bonding two wafers. Said document proposes tomold a support onto the front of one wafer, to enable the subsequentmanipulation and machining of the wafer. The support is detached fromthe wafer after it has been machined and bonded to the second wafer.

WO-A-89/11966 raises the problem of machining the surfaces of ophthalmiclenses. Said document proposes to provide plastic half-molds, avoidingany subsequent machining of the surface of the lenses; the half-moldsare used to mold lenses; it is merely necessary to machine these lensesto the shape of the frame, without machining the surface. The problemraised in said document is that of surface machining, but trimming isnot at all mentioned, or even suggested.

The prior art thus describes the problems associated with molding aviscous polymerizable material, which shrinks during the polymerization.

U.S. Pat. No. 3,881,683 discloses a method for molding ophthalmiclenses. Said document attempts to solve the problem of the thickness ofthe edges of lenses of negative power. It proposes a ring seal withseats designed to receive half-molds; the presence of these seatsreduces the risks of breakage of half-molds made of glass. Said documentalso proposes to adapt the shape of the seal to reduce the thickness ofthe edge of the lenses. In all cases, a step of machining the lens toadapt it to the shape of the frame is necessary; the document indicatesthat the method is carried out without losses on the final diameter ofthe lens. A circular lens is thus obtained, which still needs to beadapted to the shape of the frame. Said document is directed exclusivelytoward the problem of lenses of negative power, and makes no mentioneither of other lenses or of the problem of the shape of the lenses.

The invention addresses the new problem of the product obtained duringmolding. It is based on the observation that the ophthalmic lensessupplied to opticians require various operations in order to be adaptedto the shape of the frame chosen by the wearer—whether this is astandard encircling frame or a more modern frame which does not go allthe way around the lens. The lens intended to be mounted in the framemay be obtained either by machining and trimming a semi-finished glass,or by trimming a finished glass. In the first case, the operation

for surfacing the rear face is expensive; in the second case, the lensobtained is not optimized in terms of thickness.

In addition, the polymerizable material is expensive: a large amount ofthe material used during the molding is wasted, insofar as it is thendiscarded during the trimming, and possibly the surfacing.

Finally, the standard methodes pose a problem, more particularly for themanufacture of convex lenses, for which the thickness of the lens islarger at the center of the lens than at its periphery. Finished lensesare manufactured in the prior-art methodes by fixing the thickness ofthe lens at its periphery, for example at a value of 1 mm as proposed inU.S. Pat. No. 5,288,221. After trimming, especially for lenses with astrong correction, the thickness of the lens at its periphery is large.The lenses obtained have the drawback of being thicker than necessary,and thus heavier and more unattractive.

The invention makes it possible to improve the appearance of a moldedlens adapted to the shape of the frame, especially for strongcorrections; it provides a lens which, for an equal power, is thinnerthan lenses adapted to the shape of the frame by trimming finishedglasses. It ensures that the lens is of lower weight, and thus improvesthe comfort for wearers. The invention proposes a method which is lessexpensive than the one conventionally used for manufacturing such lensesfrom semifinished products. It saves on polymerizable material and makesthe manufacture by molding easier. It makes it possible to dispense withthe machining step, and to obtain a lens whose thickness is optimizedrelative to the shape of the frame, and relative to the wearer'sprescription.

To solve these new problems and to obtain these advantages, theinvention proposes a method for molding a lens adapted to a wearer'sprescription and to the shape of the frame chosen by the wearer,comprising the steps of:

-   -   supplying a first mold which is substantially in the shape of        the lens to be obtained, or of the frame to be obtained;    -   supplying a second mold;    -   molding and curing a setting material between the first mold and        a second mold;    -   removing the lens obtained from the mold.

In one embodiment, the step of supplying a first mold comprises a stepof trimming said mold.

In another embodiment, the curing step is a step of partial curing, andthe method also comprises a step of total curing of the lens.

In another embodiment, the step of molding and curing comprises a stepof assembling the molds followed by a curing step.

Advantageously, the partial curing step lasts less than 3 minutes.

Preferably, the total curing step lasts between 5 and 30 minutes.

In yet another embodiment, the method involves a step of removingmold-positioning devices after the partial curing step and before thetotal curing step.

Advantageously, the partial curing step comprises an irradiation of thesetting material, preferably by irradiation from only one side of theassembly of the first and second molds.

In one embodiment, the total curing step comprises irradiation of thesetting material, preferably by irradiation from both sides of theassembly of the first and second molds.

Preferably, the total curing step comprises the heating of the settingmaterial.

Advantageously, the demolding step comprises the application of athermal shock to the assembly of the first and second molds.

In one embodiment, the method involves a step of supplying a second moldin the shape of the lens to be obtained.

In this case, the step of supplying a second mold may comprise a step oftrimming said mold.

Preferably, the lens is an ophthalmic lens.

The invention also proposes a lens obtained according to such a method.

The invention further proposes an untrimmed molded lens of noncircularshape.

Other characteristics and advantages of the invention will emerge onreading the description which follows of the embodiments of theinvention, which is given by way of example and with reference to theattached drawings, which show:

FIG. 1 a representation in perspective of an assembly according to theinvention;

FIG. 2, representation in cutaway of the assembly of FIG. 1;

FIG. 3 a representation in top view of an assembly according to theinvention;

FIG. 4, a graph of the thickness at the edge of a lens according to theinvention.

The invention proposes, as in the prior art, to mold a lens made ofpolymerizable material between two molds; it also proposes to trim,before molding, at least one of the two molds, to give it a shape closeto that of the shape of the trimmed lens.

The invention makes it possible in this way to limit the amount ofpolymerizable material used, and to optimize the thickness of the finallens as a function of the shape of the frame and the wearer'sprescription. A saving in polymerizable material of the order of 60%relative to the standard method using a semifinished lens may beachieved.

One preferred embodiment of the invention, in which only the upper moldis trimmed, is described with reference to the figures. FIG. 2 shows arepresentation in cutaway of an assembly according to the invention;this assembly comprises a first mold 2, which is the upper mold in theexample, and a second mold 6 which is the lower mold. Between thesemolds is cast a setting material 4, for example a setting compositionsuch as those described in patent U.S. Pat. No. 5,702,825 of theApplicant. More generally, setting materials known per se, and forexample thermosetting materials, or even thermoplastics, may be used tocarry out the invention; in the latter case, the device is preferablyadapted; the technique of compression injection-molding may be used inthis case. The convex mold, generally made of metal, trimmed to theshape of the frame, may be reused.

As shown in the figure, and as emerges more clearly from FIG. 1, thesecond mold 6 is a circular mold, of the type commonly used nowadays formolding lenses; it may especially be a glass, metal, ceramic or plasticmold, such as those described in the prior-art documents mentionedabove. It has a diameter in the region of 80 mm, close to the diameterof the known lenses of the prior art.

On the other hand, the first mold 2 of FIG. 1 has a contour which issubstantially that of the lens to be obtained. “Substantially” the shapeof the lens means a shape which allows the optional subsequent trimmingof the lens to make it possible, for example, to remove small peripheralmolding defects, or alternatively a trimming of the perimeter of thelens to allow it to be mounted in a specific frame. A value of a fewmillimeters around the inner shape required by the frame is adapted. Inall cases, it is only a finishing trimming, on a few millimeters orless; it does not involve a substantial removal of material, like thatwhich is necessary to go from a circular lens with a diameter of between60 and 80 mm to a lens having the shape of the frame—which may haveBoxing sides A and B of 30 to 40 mm. The removal of material is thus atleast 1 cm.

The contour may be of any nature, and in fact depends on the nature ofthe frame chosen by the final user of the lens. The maximum size of themold between two points on its perimeter is less than the diameter ofthe second mold. The first mold is advantageously obtained by trimmingbefore molding the setting material, for example, by trimming using astandard grinding wheel for a circular mold; a grinding wheel of thetype described in patent U.S. Pat. No. 4,596,091 of the Applicant mayadvantageously be used.

Any material may be used for the first mold; insofar as the mold is tobe used only once, or otherwise only a very limited number of times, itis advantageous that its price should also be limited; one simplesolution consists in using a thermoplastic or thermosetting mold suchas, for example, a mold made of polycarbonate or poly(allyldiethyleneglycol carbonate).

The first and second molds are assembled using apparatus which is knownper se for the molding and curing of setting material; from this pointof view, the teaching of the prior art may be applied, and, for example,devices for holding in position molds of the type described in theprior-art patents mentioned above may be used. A ring seal may beincluded around the first mold, if necessary, and especially in the caseof concave lenses. If the second mold is not trimmed, it may be used asa reserve of material for the molding.

Contrary to the prior art, the invention makes it possible to obtain azero or very low thickness on the perimeter of the lens. It is possible,for example, to adjust the position of the molds such that they are incontact at one point on the periphery of the first mold, or such thatthey are 1 mm or less away from the periphery of the first mold. It isclear that the position of the molds, as in the prior art, depends onthe prescription.

The molding may be performed as follows:

introduction of the second mold;

where appropriate, insertion of a ring seal;

introduction of the setting material into the second mold; the amount ofmaerial may be calculated beforehand;

introduction of the first mold; this sequence makes it possible tosuccessfully remove the bubbles, better than an injection of thematerial between the molds; however, an injection remains possible.

The method for curing the material is performed in a manner which isknown per se. The method may also be performed in several steps, forexample:

irradiation, for example with ultraviolet rays, to set the settingmaterial;

this step may be termed as partial curing;

dismounting from the mold-holding apparatus;

total curing, for example by heating in an oven; a precise temperaturecontrol may be ensured, and the shrinkage may thus be limited, ensuringgood curing. The total curing may also be performed by ultravioletirradiation.

In a typical case, the partial curing may follow the assembly of themolds; it normally lasts less than 3 minutes, which is sufficient toensure that the assembly formed from the molds does not move. On theother hand, the total curing step may last between 5 and 30 minutes.

It is then possible to ensure removal of the lens from the mold, thislens having the power and shape required for the wearer and his frame,for example by subjecting the assembly containing the lens of theinvention to a thermal shock.

FIG. 1 shows a representation in perspective of an assembly according tothe invention; as for FIG. 1, the mold-holding apparatus has not beenshown. It is clearly seen in the figure that the first mold has a shapethat is substantially identical to the shape of a frame, whereas thesecond mold is larger in size and is designed to serve as a reservoir ofmaterial.

FIG. 3 shows a representation in top view of an assembly according tothe invention; this is the case of a lens with a power of +6.00diopters, made of a setting material with an index of 1.5, withoutoff-centering. By disregarding the effect of the thickness of the centerof the lens—which is a second-order term in the correction, a first moldhaving the shape in the figure, with a radius of curvature of 250 mm, isobtained for a second mold with a base of 8 diopters and a radius of62.5 mm. The maximum size of the first mold is 53.2 mm for the shape offrame chosen.

FIG. 4 shows a graph of the thickness of a lens manufactured using themold of FIG. 3. The thickness in millimeters is given on the y-axis andthe angle is given on the x-axis. The thickness ranges between 0 and 2.4mm. For comparative purposes, in a standard trimmed lens, the thicknessfor this prescription and this shape of frame ranges between 3.5 and 5.9mm.

For the convex mold, which is the trimmed mold in the example of thefigures, the base may be varied in steps of 0.12 diopters. For thecylinder, a step of 0.25 diopters would be acceptable. For a power rangeof 1 diopter and a cylinder range of 1 to 3 diopters, 8×12, i.e. 96different molds may thus be provided. For the concave mold, there is nochange relative to the standard manufacturing method, and the number ofmolds required to cover the range, in addition and in power, is used.

Needless to say, the present invention is not limited to the examplesand embodiments described and shown, but is susceptible to numerousvariants accessible to those skilled in the art. A wafer which must formpart of the lens may be used for one of the molds, as proposed in U.S.Pat. No. 5,288,221: in this case, the method of the invention also makesit possible to limit the amount of material.

The two molds may also be trimmed before molding.

The invention applies not only to standard prescriptions—spherical orwith a torus—but also to the manufacture of gradual or nongradualvarifocal lenses. In this case, the second mold is a varifocalaspherical mold.

1. A method for molding a non-circular lens adapted to a wearer'sprescription and to the shape of a frame chosen by the wearer,comprising the steps of: supplying a non-circular first moldsubstantially in the shape of the frame chosen by the wearer; supplyinga second circular mold; molding and curing a setting material betweenthe first mold and a second mold, a maximum size of the first moldbetween two points of its perimeter being less than a diameter of thesecond mold; removing the cured material from the first and second moldsto define the lens, and securing the lens to the frame, wherein theperiphery of the cured material is not trimmed by more than a fewmillimeters prior to securing the lens to the frame.
 2. The method asclaimed in claim 1, wherein the step of supplying a first mold comprisesa step of trimming said mold.
 3. The method as claimed in claim 1,wherein the curing step is a step of partial curing, and wherein themethod further comprises a step of total curing of the lens.
 4. Themethod as claimed in claim 1, wherein the step of molding and curingcomprises a step of assembling the molds followed by a curing step. 5.The method as claimed in claim 3, wherein the partial curing step lastsless than 3 minutes.
 6. The method as claimed in claim 3, wherein thetotal curing step lasts between 5 and 30 minutes.
 7. The method asclaimed in claim 4, wherein the step of assembling comprises usingmold-positioning devices and wherein the method further comprises a stepof removing the mold-positioning devices after the partial curing stepand before the total curing step.
 8. The method as claimed in claim 3,wherein the partial curing step comprises an irradiation of the settingmaterial from only one side of the assembly of the first and secondmolds.
 9. The method as claimed in claim 3, wherein the total curingstep comprises an irradiation of the setting material from both sides ofthe assembly of the first and second molds.
 10. The method as claimed inclaim 3, wherein the total curing step comprises heating of the settingmaterial.
 11. The method as claimed in claim 1 wherein the removing stepcomprises applying a thermal shock to the assembly of the first andsecond molds.
 12. The method as claimed in claim 1, wherein the step ofsupplying a second mold comprises a step of trimming said second mold.13. The method of claim 1, wherein the lens is an ophthalmic lens.
 14. Amethod for molding a non-circular lens adapted to a wearer'sprescription and to the shape of a frame chosen by the wearer,comprising the steps of: supplying a non-circular first moldsubstantially in the shape of the lens to be obtained; supplying asecond circular mold; molding and curing a setting material between thefirst mold and a second mold, a maximum size of the first mold betweentwo points of its perimeter being less than a diameter of the secondmold and the first mold being positioned over the second mold; removingthe cured material lens from the first and second molds to define thelens; and securing the lens to the frame, wherein the periphery of thecured material is not trimmed by more than a few millimeters prior tosecuring the lens to the frame.
 15. The process as claimed in claim 14,wherein the step of molding comprises providing the second mold in ahorizontal position.
 16. The process as claimed in claim 15, wherein thestep of molding comprises: positioning the second mold and positioningthe first mold.
 17. The process as claimed in claim 16, wherein the stepof molding comprises, after positioning the second mold, introducing thesetting material in the second mold.
 18. The process as claimed in claim16, wherein the step of molding comprises, after positioning the firstmold, injecting the setting material between the molds.
 19. The processas claimed in claim 16, wherein the step of molding comprises, afterpositioning the second mold, positioning a ring seal.
 20. The process asclaimed in claim 14, further comprising the step of providing a ringseal around the first mold.
 21. The process as claimed in claim 1,wherein the second mold is positioned 1 mm or less away from a peripheryof the first mold when molding and curing the setting material.
 22. Theprocess as claimed in claim 14, wherein the second mold is positioned 1mm or less away from a periphery of the first mold when molding andcuring the setting material.